Self-assembled mesoporous Ni_0.85Se spheres as high performance counter cells of dye-sensitized solar cells

Abstract

Transition metal chalcogenides with mesoporous structure are of special interest for application in electrocatalytic reactions due to their ample unique properties and functionalities. Nevertheless, relevant research on mesoporous Ni_0.85Se applied as electrocatalysts is rare. In this contribution, mesoporous Ni_0.85Se spheres consisting of numerous primary particles are synthesized via a facile self-assembly route. Mesoporous Ni_0.85Se spheres exhibit higher electrocatalytic activity (PCE = 7.24%) for the reduction of triiodide to iodide and lower charge-transfer resistance (R_ct = 2.40 Ω) than Ni_0.85Se nanoparticles (PCE = 6.73%, R_ct = 3.46 Ω) at the electrolyte–electrode interface in dye-sensitized solar cells (DSSCs). In order to further enhance the overall electrocatalytic performance of mesoporous Ni_0.85Se spheres, reduced graphene oxide (RGO) and single wall carbon nanotubes (SWCNTs) are introduced by facile physical mixing. RGO and SWCNT not only improved the charge transfer ability, but also yielded favorable synergistic catalytic effects with mesoporous Ni_0.85Se spheres. Simultaneously, the PCE values of Ni_0.85Se + 6 wt% RGO and Ni_0.85Se + 9 wt% SWCNT reach 7.87% and 7.74%, respectively, which are both higher than Pt (7.56%).

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Introduction

Functional nanomaterials with mesoporous nanostructures have recently attracted considerable attention due to their distinctive properties including tunable mesopore structures, large internal surface area and their easily functionalized frameworks with well-defined nanostructures.^1–5 These substantial properties enable mesoporous materials with versatile functions, such as adsorption, catalysis, drug delivery as well as energy conversion and storage.^3,6–8 In particular, mesoporous materials have exhibited excellent electrocatalytic activity because their high specific surface areas promote the interface contact between electrode and electrolyte, and the mesopores allow liquid electrolyte to easily diffuse into the electrode materials.^9–11 In recent years, considerable work has been devoted to fabricate mesoporous nanostructures of transition metal chalcogenides and make use of them as electrocatalysts due to their excellent electrocatalytic properties resulting from their combination of mesoporous structure characteristic and semiconductor framework nature.^12–15 Zhang and his co-workers prepared mesoporous Co₃S₄ nanosheets via the anion exchange reaction, in addition mesoporous Co₃S₄ nanosheets were found to be highly active and stable for electrocatalytic oxygen evolution reaction.¹⁶ Recently, Chen and his co-workers found that 3D Cu-doped CoS porous nanosheet films provided a great large number of active catalytic sites and easy accessibility toward S_n²⁻/S²⁻ electrolyte solution, leading to high-performance counter cell for quantum-dot-sensitized solar cells.¹⁷

Nickel selenides, a member of transition metal chalcogenides with Pauli paramagnetism and metallic conductivity, have exhibited abundant electrical and magnetic properties.^18–24 As an important class of nickel selenides, due to its specific stoichiometries, Ni_0.85Se owned abundant unsaturated atoms and unique electronic configuration.²⁵ Furthermore, the small difference in electronegativity between Ni (c = 1.92) and Se (c = 2.55) results in the narrow band gap of Ni_0.85Se (about 2.0 eV).^26,27 All of these outstanding features of Ni_0.85Se endow itself with many catalytic active sites, fast electron-transfer channels, as well as beneficial charge and mass transfer in electrocatalytic reaction. Thus Ni_0.85Se has shown excellent electrocatalytic performance as counter cells (CEs) of DSSCs.^28,29 So far, although several work has been done to study the nanostructures and electrocatalytic property of Ni_0.85Se, relevant research about mesoporous Ni_0.85Se applied as electrocatalytic materials is rare.³⁰

In this report, we synthesized specific stoichiometric Ni_0.85Se with the morphology of mesoporous spheres consisted of numerously primary particles, and mesoporous Ni_0.85Se spheres exhibited average pore diameter of 14.03 nm. The electrocatalytic performance of mesoporous Ni_0.85Se spheres was measured as CEs of DSSCs. Mesoporous Ni_0.85Se spheres exhibited comprehensively better electrocatalytic performance than Ni_0.85Se nanoparticles. In order to further enhance the electrocatalytic performance of mesoporous Ni_0.85Se spheres CE, different amount of RGO and SWCNT were introduced by facile physical mix. Electrochemical measurements demonstrated that both of RGO and SWCNT not only improved charge transfer ability of CEs, but also yielded favorable synergistic catalytic effect with mesoporous Ni_0.85Se spheres.³¹ More importantly, the PCE values of Ni_0.85Se + 6 wt% RGO and Ni_0.85Se + 9 wt% SWCNT reached 7.87% and 7.74%, furthermore, R_ct values were reduced to 0.68 Ω and 0.74 Ω, respectively.

Experimental section

Synthesis of mesoporous Ni_0.85Se spheres

All reagents are of analytic grade and used without further purification. The synthesis of Ni_0.85Se was carried out by a simple thermal reaction, and whole experimental flowchart is schematically illustrated in Fig. 1a. In detail, the original molar ratio between Ni(CH₃COO)₂·4H₂O and selenium powder was selected to be 1 : 1. Ni(CH₃COO)₂ (0.8 g) was dispersed in tetraethylenepentamine (TEPA) (15 ml) and stirred for 5 min, then ethylene glycol (15 ml) was added to the above solution. After stirring for another 5 min, selenium powder (0.254 g) was subsequently placed into the mixture. Lastly, 5 ml hydrazine hydrate was added dropwise. The resulting solution was put into Teflon-lined autoclave of 50 ml capacity and heated at 150 °C for 15 h. Then, the autoclave was allowed to cool to room temperature naturally. The product was washed with water and absolute ethanol to remove impurities, and then dried at 60 °C.

Fig. 1 Illustration of the synthesis process for mesoporous Ni_0.85Se spheres (a). XRD pattern of as-synthesized mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles (b).

Characterization of prepared samples

For characterizing the obtained samples, the crystallinity and composition of the samples were characterized by X-ray diffraction (XRD, D/max-2500, JAPAN SCIENCE) with Cu Kα radiation (λ = 1.54056 Å). The morphology of samples was studied by field-emission scanning electron microscopy (FE-SEM, Nanosem 430, FEI). More detailed insight into the microstructure of the sample was given by high-resolution transmission electron microscopy (TEM, Tecnai G2 F20, operating at 200 kV, FEI). The Brunauer–Emmett–Teller (BET) specific surface area was analyzed by the BET equation using a Tristar 3000 nitrogen adsorption apparatus. Fourier transform infrared spectroscopy (FT-IR) was carried on American Nicolet Instrument Co.

Test and characterization of CEs and DSSCs

The fabrication of CEs and DSSCs was listed in ESI.† All the electrochemical measurements were finished with the Zahner IM6 exelectrochemical workstation. Photocurrent–voltage curves were conducted in simulated AM 1.5 illumination (100 mW cm⁻², Trusttech CHF-XM-500 W) with a Keithley digital source meter (Keithley 2410, USA). Cyclic voltammetry (CV) was recorded with a three electrode system on the exelectrochemical workstation. Pt was used as the counter electrode, and Ag/AgCl was used as the reference electrode. An solution of 10.0 mM LiI, 1.0 mM I₂, and 0.1 M LiClO₄ in acetonitrile served as the electrolyte. CV curves were recorded in the range of −0.4 to 1.2 V at a scan rate of 25 mV s⁻¹. Electrochemical impedance spectra (EIS) analysis was conducted at zero bias potential and the impedance data covered a frequency range of 0.1 Hz to 1 MHz. The amplitude of the sinusoidal AC voltage signal was 5 mV. The analyses of the resulting impedance spectra were conducted using the software Zview 2.0. Tafel polarization measurements were employed in a symmetrical dummy cell which was used in the EIS experiments. The electrolyte was as the same of the electrolyte of DSSC. The scan rate was 20 mV s⁻¹, and the voltage range is −1.0 to 1.0 V.

Results and discussion

XRD, SEM, TEM and BJH

As shown in Fig. 1b, the phase purity of typically as-synthesized mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles were characterized by X-ray diffraction (XRD). All the primary diffraction peaks of both patterns agree with the standard data of Ni_0.85Se (JCPDS no. 18-0888, vertical line in Fig. 1b). Diffraction signals from impurities were not found in the both patterns, which indicate that only pure Ni_0.85Se phases were present in the two samples. The three strongest peaks are at about 33.04°, 44.6°, 50.24°, corresponding to the (101), (102) and (110) crystal faces respectively. XRD patterns of RGO and Fourier transform infrared (FTIR) transmission spectra of SWCNT are shown in ESI, Fig. S1.†

The morphology of the sample was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). (Fig. 2a–f) SEM image at lower magnification (Fig. 2a) proves the homogeneity of mesoporous Ni_0.85Se spheres. As shown in Fig. 2b and c, each sphere is composed of numerous primary particles with size about 50 nm. The exterior surfaces of the microspheres are not clearly smooth but contain an extensive growth of disordered interparticle porosity structure. A representative high-resolution TEM image is shown in Fig. 2f spacing of the fringes is 0.271 nm, which corresponds well to the (101) plane of Ni_0.85Se. Energy dispersion spectrum (EDS) profile indicates the existence of three elements: Cu, Ni and Se (inset in the Fig. 2f). SEM image of Ni_0.85Se nanoparticles is shown in ESI, Fig. S2.† And more detailed TEM figures of RGO and SWCNT are presented in Fig. S3 and S4.†

Fig. 2 Typical SEM (a–c), TEM (d, and e), HRTEM (f) and EDS (inset in f) images of mesoporous Ni_0.85Se spheres.

The nitrogen adsorption–desorption isotherms and porosity of mesoporous Ni_0.85Se spheres are presented in Fig. 3. The recorded adsorption and desorption isotherms of mesoporous Ni_0.85Se spheres sample display a type III isotherm with a type H3 hysteresis loop (at P/P₀ > 0.8), implying the presence of mesopores (2–50 nm in size). The inset in the Fig. 3 shows the corresponding pore size distribution calculated by the Barrett–Joyner–Halenda (BJH) method from the desorption branch, indicating a broad pore size distribution, from several nanometres to about 100 nm. The average pore diameter of mesoporous Ni_0.85Se spheres is 14.03 nm and the pore volume is 0.028 cm³ g⁻¹. The foregoing results further confirmed the porosity structure of mesoporous Ni_0.85Se spheres, which is beneficial to absorb more active species and reactants on its surface, as well as contain much reaction active sites.³²

Fig. 3 N₂ adsorption/desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of mesoporous Ni_0.85Se spheres.

Electrocatalytic performance of mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles

Electrochemical impedance spectroscopy (EIS) was carried out using a symmetric cell (CE/electrolyte/CE) consisting of two identical CEs to investigate the property of charge transfer at the CE/electrolyte interface and electrolyte diffusion in bulk solution or the film pores. As shown in Fig. 4, the EIS of mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles CEs have been investigated, in which an equivalent circuit diagram is also provided for fitting the Nyquist plots by the Z-view software, and the relevant EIS parameters are listed in Table 1. The high frequency (around 100 kHz) intercept on the real axis represents the series resistance (R_s). The semicircle in the high-frequency range (100–10 kHz) results from charge-transfer resistance (R_ct) at the electrolyte–counter electrode interface.^33,34 The R_ct value of mesoporous Ni_0.85Se spheres (2.4 Ω) was smaller than that of Ni_0.85Se nanoparticles (3.46 Ω). In addition, the R_ct value is in a contradictory relationship with the charge-transfer ability.^35,36 The lower R_ct value of mesoporous Ni_0.85Se spheres revealed its better charge-transfer ability in comparison with Ni_0.85Se nanoparticles.

Fig. 4 Nyquist plots for symmetric cells fabricated with mesoporous Ni_0.85Se spheres, Ni_0.85Se nanoparticles and Pt.

Table 1 Corresponding EIS, Tafel polarization and CV parameters of the DSSCs assembled with mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles CEs

CEs	Rs (Ω)	Rct (Ω)	Epp (mV)	JA (mA cm^−2)	J0 log (mA cm^−2)	Jlim log (mA cm^−2)
Mesoporous Ni0.85Se spheres	20.78	2.40	351	1.91	1.96	0.59
Ni0.85Se nanoparticles	20.61	3.46	404	0.80	1.84	0.55
Pt	21.60	1.28	429	1.81	2.18	0.85

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In order to further evaluate the electrocatalytic performance, the cyclic voltammetry (CV) was measured in a three-electrode system. As shown in Fig. 5, all CEs have typical curves with two pairs of redox peaks and two cathodic peaks, which are related to the two-step process of I₃⁻ reduction denoted in eqn (1) and (2):

I₃⁻ + 2e ↔ 3I⁻ (1)

3I₂ + 2e ↔ 2I₃⁻ (2)

Fig. 5 Cyclic voltammograms for iodide/triiodide redox species of mesoporous Ni_0.85Se spheres, Ni_0.85Se nanoparticles and Pt.

Peak-to-peak separation (E_pp) in lower potential is negatively correlated with the standard electrochemical rate constant and has positive correlation with overpotential losses.^37,38 As the Fig. 5 shows, the peak-to-peak separation (E_pp) values of mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles were about 351 mV and 404 mV. The E_pp trend revealed the better catalytic activity for reducing I₃⁻ of mesoporous Ni_0.85Se spheres than Ni_0.85Se nanoparticles. As mentioned above, mesoporous structures promote the interface contact between electrode and electrolyte, and contribute to the diffusion of liquid electrolyte into the inside of mesoporous Ni_0.85Se spheres. Therefore, the rate of electrocatalytic reaction the catalytic activity at the electrolyte–mesoporous Ni_0.85Se spheres electrode interface was improved and faster than Ni_0.85Se nanoparticles. Consequently, the peak current density (J_A) of mesoporous Ni_0.85Se spheres CE (1.74 mA cm⁻²) in lower potential is higher than that of Ni_0.85Se nanoparticles CE (0.78 mA cm⁻²), and the J_A is positively correlated with the electrocatalytic reaction rate.^39,40

Fig. 6 shows the Tafel polarization curves of different CEs which were measured by the dummy cells. The polarization zone of Tafel polarization curve corresponds to the high frequency region of Nyquist plots, and diffusion zone relates to the low frequency region in EIS.²⁶ In the polarization zone of Tafel polarization curve, as the reaction velocity of I₃⁻ reducing to I⁻ unmatches with the transfer speed of photo-generated electrons, electrons or ions accumulate in CEs and generate the overpotential, then electrochemical polarization appears and the voltage of DSSC devices will reduce. Herein, the exchange current density (J₀) in the polarization zone corresponds with R_ct of Nyquist plots. Accordingly, as to diffusion zone, the inconformity between diffusion rate of iodide ions and reaction velocity of I₃⁻ to I⁻ induces the concentration polarization. Similarly, the limiting diffusion current density (J_lim) can represent diffusion coefficient of I₃⁻ and I⁻. As shown in Table 1, similar with the R_ct values, the calculated J₀ (1.96 log (mA cm⁻²)) and J_lim (0.59 log (mA cm⁻²)) values (Fig. S5†) of mesoporous Ni_0.85Se spheres were both higher than those of Ni_0.85Se nanoparticles (J₀ = 1.84 log (mA cm⁻²), J_lim = 0.55 log (mA cm⁻²)). J₀ and J_lim both agreed well with the results of EIS experiments, and demonstrated the faster charge transfer speed and diffusion rate of mesoporous Ni_0.85Se spheres than those of Ni_0.85Se nanoparticles.⁴¹

Fig. 6 Tafel polarization curves for symmetric cells fabricated with mesoporous Ni_0.85Se spheres, Ni_0.85Se nanoparticles and Pt.

Current density–voltage (J–V) measurements under the AM 1.5 G condition were carried out to compare the comprehensive electrocatalytic properties of mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles. Fig. 7 shows the J–V curves for DSSCs using the above three CEs, and corresponding photovoltaic parameters of open-circuit voltage (V_oc), short-circuit current (J_sc), fill factor (FF) and PCE are summarized in Table 2. Mesoporous Ni_0.85Se spheres owned higher J_sc (14.40 mA cm⁻²), FF (0.67) and PCE (7.24%) than those of Ni_0.85Se nanoparticles (J_sc = 14.00 mA cm⁻², FF = 0.65, PCE = 6.73%), which demonstrated comprehensively better electrocatalytic properties of mesoporous Ni_0.85Se spheres than Ni_0.85Se nanoparticles. This advantage can be visually attributed to the higher current intensity of mesoporous Ni_0.85Se spheres than Ni_0.85Se nanoparticles, and in Fig. 7 the J–V curve of mesoporous Ni_0.85Se spheres overall located above that of Pt. Simultaneously, higher FF value of mesoporous Ni_0.85Se spheres, revealing their better utilization efficiency of photogenerated electron than Ni_0.85Se nanoparticles.⁴² As mentioned above, mesoporous structures of mesoporous Ni_0.85Se spheres promote the interface contact between electrode and electrolyte, and contribute to the collision among I₃⁻ and catalytic active sites. EIS, CV and Tafel polarization already demonstrated that mesoporous Ni_0.85Se spheres exhibited better catalytic activity and charge transfer ability, which accelerated the rate of electrocatalytic reaction and further improved the PCE value.

Fig. 7 Photocurrent density–voltage characteristics of DSSCs with mesoporous Ni_0.85Se spheres, Ni_0.85Se nanoparticles and Pt.

Table 2 Corresponding photovoltaic parameters of the DSSCs assembled with mesoporous Ni_0.85Se spheres and Ni_0.85Se nanoparticles CEs

CEs	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)
Mesoporous Ni0.85Se spheres	0.75	14.40	0.67	7.24
Ni0.85Se nanoparticles	0.74	14.00	0.65	6.73
Pt	0.76	14.80	0.67	7.56

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Enhanced electrocatalytic performance of mesoporous Ni_0.85Se spheres with RGO and SWCNT

In order to improve the electrocatalytic performance of mesoporous Ni_0.85Se spheres and avert the adverse effect of the poor interdomain electron transport among spheres, introducing carbon materials (RGO or SWCNT) to take advantage of their high specific surface area and extremely high electrical conductivity should be an ideal idea.⁴³ Therefore, RGO and SWCNT with different amount were introduced by facile physical mix to enhance electrocatalytic activity of mesoporous Ni_0.85Se spheres CE. The information about RGO and SWCNT is shown in ESI.† On one hand, EIS experiments (Fig. 8a and b and S6†) were performed to measure the effect of mixing carbon materials with mesoporous Ni_0.85Se spheres on charge transfer ability, and detailed EIS parameters are summarized in Table 3. As mentioned, R_ct value was 2.40 Ω for Ni_0.85Se, when mesoporous Ni_0.85Se spheres mixing with RGO (NRCs), the R_ct values of Ni_0.85Se + 3 wt% RGO, Ni_0.85Se + 6 wt% RGO and Ni_0.85Se + 9% RGO were 0.82 Ω, 0.68 Ω and 0.86 Ω, respectively. As to Ni_0.85Se + SWCNT CEs (NSCs), the mixtures of Ni_0.85Se with 3 wt% (R_ct = 0.93 Ω), 6 wt% (R_ct = 0.90 Ω) and 9 wt% (R_ct = 0.74 Ω) SWCNT also exhibited lower R_ct than that of mesoporous Ni_0.85Se spheres. Furthermore, all of the R_ct values of NRCs and NSCs are lower than that of Pt CE (1.28 Ω). Obviously, the addition of RGO and SWCNT greatly enhanced the charge transfer ability of mesoporous Ni_0.85Se spheres. In consideration of the reason, it may due to that the addition of RGO and SWCNT not only improved the charge transfer between Ni_0.85Se spheres, but also promoted the transfer of photo-generated charge from CEs materials to electrolyte molecules.^44,45 On the other hand, Tafel polarization and CV experiments of NRCs and NSCs were also carried out to demonstrate the enhancement effect of RGO and SWCNT on the electrocatalytic property of mesoporous Ni_0.85Se spheres CE, and relevant information was listed in ESI Fig. S7–S9.†

Fig. 8 Nyquist plots (a and b) and photocurrent density–voltage characteristics (c and d) of DSSCs fabricated with NRCs, NSCs and Pt.

Table 3 Corresponding photovoltaic and electrochemical impedance parameters of the DSSCs assembled with various CEs

CEs	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)	Rs (Ω)	Rct (Ω)
Ni0.85Se + 3 wt% RGO	0.74	15.60	0.66	7.58	20.11	0.82
Ni0.85Se + 6 wt% RGO	0.76	15.20	0.69	7.87	20.25	0.68
Ni0.85Se + 9 wt% RGO	0.74	15.20	0.68	7.62	19.58	0.86
RGO	0.75	11.20	0.67	5.61	19.21	7.16
Ni0.85Se + 3 wt% SWCNT	0.76	14.00	0.71	7.45	20.62	0.93
Ni0.85Se + 6 wt% SWCNT	0.75	14.80	0.68	7.50	20.52	0.90
Ni0.85Se + 9 wt% SWCNT	0.76	14.80	0.69	7.74	20.43	0.74
SWCNT	0.77	11.60	0.68	6.07	18.32	6.46

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Photoelectric conversion efficiencies of NRCs and NSCs (Fig. 8c and d and S10†) were measured to evaluate the overall electrocatalytic effect between mesoporous Ni_0.85Se spheres and carbon materials. As to the mesoporous Ni_0.85Se spheres CE, PCE value was 6.73%. With the addition of RGO, the photovoltaic performances (Table 3) have been improved substantially, and PCE values of Ni_0.85Se + 3 wt% RGO (7.58%), Ni_0.85Se + 6 wt% RGO (7.87%) and Ni_0.85Se + wt% RGO (7.62%) were higher than those of mesoporous Ni_0.85Se spheres and Pt (7.56%) CEs. SWCNT also evidently raise the PCE of mesoporous Ni_0.85Se spheres CE, and PCE values of Ni_0.85Se + 3 wt% SWCNT, Ni_0.85Se + 6 wt% SWCNT and Ni_0.85Se + wt% SWCNT were 7.45%, 7.50% and 7.74%, respectively. As the photovoltaic parameters in Table 3 show, NRCs and NSCs both showed higher values of FF and much higher J_sc values than those of mesoporous Ni_0.85Se spheres, which represented that both of RGO and SWCNT not only promoted the charge transfer in composites but also enhanced the utilization efficiency of photo-generated charge.³⁰

As mentioned in the above results, different amount of RGO or SWCNT exhibited different effect on the electrocatalytic property of mesoporous Ni_0.85Se spheres CE. In consideration of reaction mechanism, the lower percentage of RGO or SWCNT than the ideal amount in composites cannot reach the requirement of charge-transfer, and the catalytic activity of the composites cannot get fully exploited. This condition can be demonstrated by the results of EIS and CV experiments. When the amount of RGO or SWCNT is more than the ideal amount, the overmuch RGO or SWCNT will decrease the amount of active sites in the basal plane for I₃⁻ electrocatalysis. And weak adhesion of overmuch RGO or SWCNT to conductive substrate hinders the reception of photo-generated electrons from photoanode, which is adverse to charge-transfer and catalytic reaction.⁴⁶ Herein, Ni_0.85Se + 6 wt% RGO and Ni_0.85Se + 9 wt% SWCNT CEs obtained the best charge-transfer ability and electrocatalytic performance in NRCs and NSCs, respectively.

Conclusions

In conclusion, we have demonstrated a facile self-assembly method to fabricate mesoporous Ni_0.85Se spheres consisted of numerous primary particles. Their mesoporous structures and large internal reaction area endow mesoporous Ni_0.85Se spheres with excellent performance as CEs of DSSCs. It is evidenced that mesoporous Ni_0.85Se spheres owned overall better electrocatalytic activity for the reduction of I₃⁻ than Ni_0.85Se nanoparticles. On the other hand, the addition of RGO and SWCNT substantially improved the interdomain electron transport among Ni_0.85Se spheres and accelerated the velocity of electrocatalytic reaction. Simultaneously, the most appropriate amount of carbon materials resulted in the optimal effect for enhancing the electrocatalytic performance in DSSCs.

Acknowledgements

This work was supported by the National Science Foundation of China (No. 21271108), the Ministry of Science and Technology (Grant 2014CB932001), the China-U.S. Center for Environmental Remediation and Sustainable Development, the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program) (Grant 2014CB932001), National Natural Science Foundation of China (No. 21425729).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11085a

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